Yuchun Zhai

Preparation and sintering properties of zirconia-mullite-corundum composites using fly ash and zircon

Zirconia-mullite-corundum composites were successfully prepared from fly ash, zircon and alumina powder by a reaction sintering process. The phase and microstructure evolutions of the composite synthesized at desired temperatures of 1 400, 1 500 and 1 600 °C for 4 h were characterized by X-ray diffractometry and scanning electronic microscopy, respectively. The influences of sintering temperature on shrinkage ratio, apparent porosity and bulk density of the synthesized composite were investigated. The formation process of the composites was discussed in detail. The results show that the zirconia-mullite-corundum composites with good sintering properties can be prepared at 1 600 °C for 4 h. Zirconia particles can be homogeneously distributed in mullite matrix, and the zirconia particles are around 5 μm. The formation process of zirconia-mullite-corundum composites consists of decomposition of zircon and mullitization process.

A green process for recovering nickel from nickeliferous laterite ores

Abstract A novel process was proposed for recovering nickel from nickeliferous laterite ores. First of all, silicon and magnesium were removed from lateritic ore by high concentration sodium hydroxide leaching and carbonation respectively, so as to enrich nickel. Then the method of ammonia carbonate leaching was adopted to recover nickel from the carbonized slag, and the remaining residue was used as a raw material for recovering iron. The effects of temperature, ammonia carbonate concentration, liquid/solid ratio and stirring speed on the recovery of nickel were examined. When the leached residue reacted with ammonia carbonate (6 mol·L −1 ) in a ratio of liquid-to-solid of 5:1 at 60 °C for 150 min at the stirring speed of 300 r·min −1 , approximate more than 95% nickel was recovered. During the whole process, there was no contamination produced and the chemical raw materials were recycled, thus the process was a green technology that having good social benefit.

Effect of water vapor from power station flue gas on CO2 capture by vacuum swing adsorption with activated carbon

Abstract Due to the high absolute humidity of real flue gas, activated carbon, a hydrophobic adsorbent, was used to selectively adsorb CO2 by vacuum swing adsorption in this study. The objective of this work is to study the feasibility and advantage of CO2 capture along with simultaneous moisture removal by activated carbon and the effect of H2O on CO2 capture from wet flue gas streams. Through experiment and analysis, the “S” shape isotherms of water indicated water was easier to be desorbed from activated carbon. Then a cone shape model was proposed to depict water distribution inside the adsorption bed. As a consequence, water vapor hardly influenced the CO2 capture performance. Moreover, the process can be operated under a relatively high vacuum pressure and short evacuation time. The preliminary results showed that our one-bed VSA process could yield a good CO2 recovery of over 80% and a reasonable purity of 43%.

Desiliconization kinetics of nickeliferous laterite ores in molten sodium hydroxide system

Abstract A novel process was proposed for treating nickeliferous laterite ores with molten sodium hydroxide. The effect on silicon extraction caused by the factors, such as stirring speed, reaction temperature, particle size and NaOH-to-ore mass ratio, was investigated. The results show that increasing stirring speed, reaction temperature and NaOH-to-ore mass ratio while decreasing particle size increases silicon extraction rate. The desiliconization kinetics of nickeliferous laterite ores in molten sodium hydroxide system was described successfully by chemical reaction control model. The activation energy of the desiliconization process was found to be 44.01 kJ/mol, and the reaction rate based on a chemical reaction-controlled process can be expressed as: 1–(1–α) 1/3 = 27.67exp[−44 010/( RT )] t .

Reaction kinetics of roasting high-titanium slag with concentrated sulfuric acid

A novel method of roasting high-titanium slag with concentrated sulfuric acid was proposed to prepare titanium dioxide, and the roasting kinetics of titania was studied on the basis of roasting process. The effects of roasting temperature, particle size, and acid-to-ore mass ratio on the rate of roasting reaction were investigated. The results showed that the roasting reaction is fitted to a shrinking core model. The results of the kinetic experiment and SEM and EDAX analyses proved that the reaction rate of roasting high-titanium slag with concentrated sulfuric acid is controlled by the internal diffusion on the solid product layer. According to the Arrhenius expression, the apparent activation energy of the roasting reaction is 18.94 kJ/mol.

Preparation and characterization of ultrafine zinc oxide powder by hydrothermal method

Abstract With Zinc acetate and sodium hydroxide as raw materials, while polyethylene glycol employed as dispersant agent, ultrafine zinc oxide powder was synthesized by hydrothermal method. Influence of NaOH concentration on morphology of ZnO powder was studied. The as-synthesized ZnO powder looked like flower cluster and consisted of microrods with hexagonal morphologies. The crystal structure and optical property of the as-prepared powder were also characterized using XRD, UV-visible absorption spectrum and photoluminescence spectrum. The results indicate that ZnO powder is of hexagonal wurtzite structure and well crystallized with high purity. There is a strong excitation absorption peak at 300 nm in UV-visible absorption spectrum and blue shift exists obviously. The optical property of ZnO powder is excellent.

Kinetics of SiO2 leaching from Al2O3 extracted slag of fly ash with sodium hydroxide solution

Abstract Kinetics of SiO 2 leaching from Al 2 O 3 extracted slag of fly ash with sodium hydroxide solution was studied. The effect of leaching temperature, mass ratio of NaOH to SiO 2 and stirring speed on SiO 2 leaching rate was investigated. The results show that increasing leaching temperature, mass ratio of NaOH to SiO 2 and stirring speed increases SiO 2 leaching rate. The SiO 2 leaching rate is 95.66% under the optimized conditions. There are two stages for the SiO 2 leaching process, and the leaching reaction is very rapid in the first stage but quite slow in the second stage. The whole leaching process follows the shrinking core model, and the outer diffusion of no product layer is the rate-controlling step. The activation energies of the first and second stages are calculated to be 8.492 kJ/mol and 8.668 kJ/mol, respectively. The kinetic equations of the first and the second stages were obtained, respectively.

Thermodynamics and kinetics of extracting zinc from zinc oxide ore by the ammonium sulfate roasting method

Thermodynamic analyses and kinetic studies were performed on zinc oxide ore treatment by (NH4)2SO4 roasting technology. The results show that it is theoretically feasible to realize a roasting reaction between the zinc oxide ore and (NH4)2SO4 in a temperature range of 573–723 K. The effects of reaction temperature and particle size on the extraction rate of zinc were also examined. It is found that a surface chemical reaction is the rate-controlling step in roasting kinetics. The calculated activation energy of this process is about 45.57 kJ/mol, and the kinetic model can be expressed as follows: 1 − (1 − α)1/3 = 30.85 exp(−45.57/RT)·t. An extraction ratio of zinc as high as 92% could be achieved under the optimum conditions.

Extraction of alumina from fly ash by ammonium hydrogen sulfate roasting technology

Abstract NH 4 HSO 4 roasting technology was used for preparing Al 2 O 3 from fly ash. First, Al and Fe were extracted from fly ash by NH 4 HSO 4 roasting and deionized water leaching. Then, the Al and Fe in the leached liquid were precipitated by adding NH 4 HCO 3 solution. After the mixed precipitations of Al(OH) 3 and Fe(OH) 3 were leached by NaOH solution, the NaAl(OH) 4 solution was decomposed by carbonation. Finally, the pure Al(OH) 3 was calcined to α-Al 2 O 3 . The optimal conditions of the whole technology were determined by experiments. The quality of α-Al 2 O 3 product is up to the technical indicator of YS/T 274-1998 standard.

Thermodynamics and kinetics of alumina extraction from fly ash using an ammonium hydrogen sulfate roasting method

A novel method was developed for extracting alumina (Al2O3) from fly ash using an ammonium hydrogen sulfate (NH4HSO4) roasting process, and the thermodynamics and kinetics of this method were investigated. The thermodynamic results were verified experimentally. Thermodynamic calculations show that mullite present in the fly ash can react with NH4HSO4 in the 298–723 K range. Process optimization reveals that the extraction rate can reach up to 90.95% when the fly ash reacts with NH4HSO4 at a 1:8 mole ratio of Al2O3/NH4HSO4 at 673 K for 60 min. Kinetic analysis indicates that the NH4HSO4 roasting process follows the shrinking unreacted core model, and inner diffusion through the product layer is the rate-controlling step. The activation energy is calculated to be 16.627 kJ/mol; and the kinetic equation can be expressed as 1 − (2/3)α − (1 − α)2/3 = 0.0374t exp[−16627/(RT)], where α is the extraction rate and t is the roasting temperature.